Thermionic vacuum tube



June 10, 1930. c:.v w. CARTER, JR

1,762,212 THERMIONIC vAcpuiu TUBE Filed Dec. 20, 1924 2 Sheets-Sheet l Current I L Voltage INVENTOR I 7 5 0. WCarter;

ATTORNEY June 10,1930.

Current :7 INVENTOR C, WCarteI;

7 z Sheets-Sheet 2 Patented June 10, 1930 V UNITED STATES, PATENT OFFICE CHARLES W. CARTER, JR; OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK mnnnivrronrc vAcUuM T BE Application filed December 20,-1 924f'Seria1. No. 757,268.

a characteristic to suit special circumstances,

as for example, to increase its curvature for a rectifier or to decrease itscurvature for an amplifier. These and various other objects of my invention will become apparent'on consideration of a limited number of examples of practice in accordance with the invention, which I have chosen to disclose in'this speci-. fication. It will be understood that the invention is defined in the appended claims and .20 that the following description relates more particularly to the examples of the invention here set forth to illustrate its character.

Referring to the drawings, Figure-1 is a diagramof a two-electrode thermionic vacuum tube and its'ac companying circuits; Fig;

2 is a diagram of a three-electrode vacuum tube and its associated circuits; Figs. 3 and 4: are current voltage diagrams," and Figs. 5,

6, 7 and 8 are plots of minimumsaturati'on potentials along the length of a filament cathode.

A two-electrode thermionic vacuum tube of simple design is shown in Fig. 1. VVithinithe evacuated vessel 21 thereis a cathode in the form of a filament l1-12-and a corresponding anode 1 4 in the form of a cylindrical shell having the cathode in its axis. The cathode 11-12 is heated by the current due to the battery 13', and the battery T 16'is connected 49 between the cathode 1112and the anode 14 at the point 15 so as tomake the cathode negative. Accordingly, as electrons are'emitted from the cathode ll-12, they (or: partrof them) are drivenacross tot theianode 14' and 5 thus a currentis caused to flow in the circuit of thebattery 1'6. v

Due to the spa'c'e' charge efi'ect'theelec trons near the cathode 11' 12 have a tendency tore-enter it, and the voltage cram battery 16 must bepushed up abo've'a certain critical effects of,

voltage-to get all the electrons across from thevfilament-cathode 1 112 to the anode 14. When this critical voltage,'cal led herein the! minimum saturation potential, is 'attainedy the-current remains constant with further increase of voltage. The current-voltage relation will be of the character shown by the curve OAC in Fig. 4. I

This curve is based on the assumption that i the filament 1112is of uniform potential, whereas in fact there will obviously be a drop inpotential' along its length due to the heating voltage, also other simplifying assumptions are involved, as that the rate of emission is uniform over the entire filament surface, whereas in fact the rate of emission is nonuniform for two-principal reasons: vari the greater is the minimum saturation volt;

age. Thus, if the characteristic has the form OAC as in Fig; 3,'for a certain filament tem- Y ation of the work function over the surface,

perature, then an increase of the "filament tema I I perat-ure will change'the characteristic to the" lociusOBC. v V p On account :of the lower temperature ofthe' ends of the filament. due to conduction of'he'at into the leadsas already mentioned, andbecause of impurities," inhomogeneities, .etc., the

minimum saturation potentials for' difierent parts of the. filament will be diiferent. Thus if the horizontaliline l112in Fig. 6 represents the filamentythen the corresponding ordinatesfofeither curve may bet'aken as representing tlfecOrr'esponding minimum saturation potentials for the respectivepoints of thefilament, the two curves being'drawn for two'di'fierent heating voltages? The poten{ tial due to the filament heating h'attfery 'l 3' is 7 shown in'Fig; 7, and the combination 'ofithe Y Eigs'.g6{and gives the curjv'e": of

Fig-8.

. To obtaifnan understanding of the effect of. the current voltage "characteristic 'o fthe' tube due to] the lower minimum saturation potentials toward the ends of" the filament,-

O i that the shape of the characteristics is dep the respective ordinates of the latter range averaging the corresponding ordinates of the other two curves. Thus it will be seen that the composite filament has the upper sloping portion of its characteristic displaced tothe right.

llow if the filament is composed of a large number of portions of difierent saturation potentials the displacement to the right will be in far down on the curve and will increase farther along up the curve so that the upper partof the sloping part of the curve will be pushed more and more to the right and will be correspondingly straightened. This is illustrated by comparing the curves 25, 27 and 29 of F ig. 4 with the curve 23. These curves are constructed for certain ideal cases represented in Fig. 5 where the "ordinates represent saturation potentials for the successive points along the filament 1112. When the saturation potential is uniform along the entire length of the filament we have the line 23-24 in Fig. 5, for which the current voltage curve is 23 in Fig. 4.

Next assume that the minimum saturation potential drops ofi' at the ends asshown by the line 11252612 in Fig. 5. This gives the curve 25 in Fig. 4. A more pronounced transition at the ends of the filament, as indicated by the line 112728-12 in Fig. 5, gives the curve 27 in Fig. 4, and the extreme case represented by 11 29-12 in Fig. 5

gives the curve29 in Fig. 4. In practice these characteristics are shifted to the right by an amount equalv approximately to half the heating voltage and a continuous transition to saturation is provided by the heating voltage, in addition to the change in curvature caused by the variation of the minimum saturation potential. 5 It is evident, however,

termined by the variation along the filament of the minimum saturation potential, and that the wider the limits between which the minimum saturation potential varies, the greater will bethe departure from the char acteristic'pf the ideal equipotential, isothermal filament.

In many practical tubes with a uniformly constructed-filament,such an extreme gradation as that shown in 11'2912 in Fig. 5

would be impossible simply by cooling the ends by conduction of heat into the leads.

Various materials used as filaments for thermionic vacuum tubes emit electrons at difierent rates at the same temperature. Furthermore many thermionic vacuum tubes have their filaments coated with certain oxides, such as calcium oxide, these coatings emitting electrons at various rates at the same temperature. Evidently, other things being equal, a filament with high emissivity will require a higher minimum saturation potential than a filament with low emissivity. The shape of the minimum saturation potential curve, Fig. 8, accordingly may be varied by application of coatings of diflerent emissivities on different parts of the filament. In particular the effect of cooling at' the ends may be accentuated by application of a coating of high emissivity to the hottest part of the filament coatings, of less emissivity .to cooler parts, and least emissivity to the coolest parts of the filament. The resulting variation of minimum saturation potentials is consequently greater, enhancing the straightness of the characteristic. Conversely, by applying coatings of high emissivity to the coolest portions and lowemissivity to the hottest portions of the filament, the efiect of cooling at the ends may be compensated, giving an approach to a constant minimum saturation potential for the whole filament, and consequently enhancing the curvature of the characteristic.

Thus far the discussion has related to a two-electrode vacuum tube as in Fig. 1. In the three-electrode tube, as in Fig. 2, there is a third electrode or grid 20 usually placed between the filament 1112 and the anode 14, and the potentials on the grid and anode are in general difi erent. Moreover, although the potential on the grid varies, it is usually kept negative all the time by means of the battery 17 so that no current flows to the grid. Under these circumstances the three-electrode tube can be treated as a two-electrode tube with a hypothetical anode voltage V given by the equation where V is the anode voltage, V is the grid voltage, and a and ,e are constants for any partlcular tube. 1

In case the three-electrode tube is used as an amplifier, it will generally be desirable that the relation shall be linear between the output current in circuit 22 and the input voltage 1n circuit 18. Referring to Fig. 4, the upper sloping part of the characteristic 25 is nearly a stralght line and the intermediate sloping part of the characteristic 27 is very nearly stra1ght, and a portion a little lower down for the characteristic 29 is substantlally straight. If the tube is adjusted so that 1t works as an amplifier on the straight sloping portion of its characteristic, it will ampllfy wlthout distortion, and by coatin r the filament-cathode so as'to accentuatethz lower emissivity at the cooler parts of the filament, the more advantageous characteristics 25, 27 or 29 can be obtained instead of the ideal characteristic 23. r

In case a vacuum tube of two electrodes or three electrodes is to be used as a rectifier or detector or modulator, a curved characteristic I becomes desirable, and in sucha case if the cooling at the ends of the filament gives a 30 characteristic more like 25 or 27 or 29 in Fig. 4, the eflect may be compensated by coating the cooler parts of the filament with material of higher emissivity so as to bring the characteristic back to or toward the form shown at 23 in Fig. 4, with substantial curvature along its length. .7

Besides modifying the shape of the characteristic by varying the character of the fila-- ment coating along its length,somewhat the same effect may be produced to a degree by varying the cross section of the filament. If the end portions of the filament are of greater I cross section than the intermediate portion they will be at lower temperature on that'account and the same eifect willbe secured as if a filament of uniform cross section were coated with material of less emissivity toward its ends. Reciprocally, if the cross section of the filament is greater near its middle 30 part, then on that account the filament will be,

hotter near its ends and the efiect will be somewhat the same as if a uniform filament were coated with material of greater emissivity toward its ends. V

I claim:

v A thermionic high vacuum electron-dis- 7' charge tube having its cathode in the form of an elongated filament coated with materials of emissivity value varying from zone to zone 40 along its length whereby the current-voltage characteristic is given a desired shape. I

In testimony whereof, I1 have signed my name to this specification this 19th day of December 1924. I CHARLES W. CARTER, JR.

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